A (Not So) Scientific Look at the Randomness of DNA

I have now tested myself, my daughter, my parents, my maternal first cousin, my maternal grandfather's first cousin, my maternal grandfather's half nephew, my maternal grandmother's sister, and three distant cousins of my father. Along with several of my correspondents who have also tested that I know the relationship, I already had some theories about the randomness of DNA.

There are multiple blog posts on the X chromosome DNA. How entire sequences seem to pass from one generation to the next. How the distribution of that DNA isn't scientific. But there isn't much, except for a few posts on "sticky" DNA that addresses the other 22 chromosomes.

I knew for instance, my mother and her half first cousin seem to have gotten the same but unequal parts of DNA from their grandfather. Mom tended to get more of the Hager side, and her cousin more of the Collins side. I knew that mother's colonial Massachusetts DNA and the DNA from her Wallen ancestors was "sticky". She matched larger segments than the relationship should allow. I knew that in a cases beyond 3rd cousin, how much DNA I got from my parents really depended on the number of segments. If it was one segment, I either got it or I didn't. If though it was several, I more often than not got at least one segment, if not all of them.

I was anxious to see just how my daughter's DNA looked. Would she share more from one grandparent than the 25 %? I suspected she would. So here is a look at my daughter's DNA mostly from Gedmatch (I know some of the math seems off with my mother's)

Relationship to my daughter	Total cM’s	Percent of DNA of both parents	Percent of DNA as my Half contributed (percent of my 50 %)	My DNA Total cM’s
Mother	3581/194.7 (X)	50	100	Na
Grandfather	2001.8/154.2(x)	27.95/39.6 (x)	55.9/79.2 (x)	3552/194.2
Grandmother	1586.4/47(x)	22.15/12(x)	44.3/24.1 (x)	3581/194.7
Maternal GG Aunt	609	8.5	17	1247.2
Maternal 1C1R	429	6	12	1039.7
Maternal 1C3R	74.4/16.6	1/ 4.2 (x)	2.1/8.5 (x)	233.5/83.7
Maternal ½ 1C2R	35.1	0.49	0.98	214.1
Maternal 3C1R #	162.1	2.26	4.5	220.3
Maternal 2C3R	55.1	0.77	1.53	63.2
Maternal 3C1R	84.6	1.32	2.36	84.9
Maternal 1C2R *	486	6.79	13.6	735
Maternal 1C2R *	372	5.195	10.39	696
Paternal 2C1R *	231	3.22	na	na
Paternal  2C *	277	3.87	na	na

·         * Total DNA from ancestry.com. Remaining Kits are from Gedmatch results.

·         # is a double cousin. Both my and her great grandparents (one set) were siblings married to siblings. 

S    So how does my great Aunt fair? Well, I suppose because she's 86, she has much fewer matches. But here is the percentages she shares with my mother, my cousin, and me (see above for my daughter). She shares 2006.1 cM's with my mom or roughly 28% of my mom's DNA. She shares 1247.2 cM with me or roughly 17.4 % of the DNA with me. I share none of her X chromosome (she's my maternal grandmother's sister) but my mom shares 61.1 cM or roughly 39 % of her maternal X chromosome. My first cousin (who is her great nephew) shares 998.9 cM or roughly 13.9 % DNA with our Great Aunt. I share more DNA with my great Aunt than my first cousin, and my first cousin shares slightly more with me than with my great Aunt, even though in both cases the tables show we should share an average of 12.5 %.